Processes and apparatuses for manufacturing fiber-drawing preforms are known from U.S. Pat. No. 4,445,918, U.S. Pat. No. 4,816,050 and U.S. Patent Publication No. 2003/0084685, which are hereby incorporated by reference in their entirety. In addition Japanese Patent No. 59-121129, which is hereby incorporated by reference, relates to a method for manufacturing a glass body for optical fibers in which fine powder layers of glass are formed on the inner walls of glass pipes.
Preforms for optical fibers may be formed using an internal chemical vapor deposition (CVD) technique, wherein doped or undoped glass layers are deposited onto the inner surface of a hollow glass substrate tube. To carry out such a deposition, reactive gases are supplied into one side of the substrate tube (i.e., the supply side) forming a glass layer onto the interior surface of the substrate tube as a result of special process conditions. An energy source is moved back and forth along a specific part of the length of the substrate tube for forming a glass layer. The energy source (e.g., a plasma generator) supplies high-frequency energy, thereby generating a plasma in the interior of the substrate tube. Under such plasma conditions, the reactive glass-forming gases will react (e.g., the plasma CVD technique). It is also possible, however, to supply energy in the form of heat, particularly by using burners, on the outer side of the substrate tube or via a furnace, which surrounds the substrate tube. Typically, the energy source is moved back and forth with respect to the substrate tube.
A drawback of the foregoing techniques is that, because of the reciprocating movement of the energy source, defects near the points of reversal may develop in the layers deposited on the interior surface of the glass substrate tube. The defects are typically called “taper,” which may include geometric taper and optic taper. The term “geometric taper” describes the fact that the thickness of the total deposition (i.e., all of the glass layers) is not constant along the length of the tube. The term “optic taper” describes the fact that the optical properties are not constant along the length of the preform, and that consequently the optical properties of the optical fibers obtained from such a preform are not constant. Optic taper is mainly determined by differences in the refractive index, or refractive index profiles, along the length of the preform.
In addition to an adequate control of the geometric taper, with regard to realizing adequate control of the optical properties of the fibers to be formed, it is also desirable that the differences in the longitudinal refractive index profile be as small as possible over a substantial length of the preform. Typically, during deposition processes the length of the deposition zones located at either end of the substrate tube, also referred to as “end tapers,” may include about 15 percent of the total length of the substrate tube. The presence of such “taper” leads to an axially non-uniform core cross-section. In particular, the taper forms a region in which the optical and/or geometrical properties of the preform are not uniform. The non-uniformity will result in degradation of the transmission characteristics of the optical fiber. Therefore, the “taper” regions in the preform are generally not used for manufacturing optical fibers. Because such “taper” regions form a significant part of the length of the preform rod, the total fiber length to be obtained from a preform is somewhat limited.
In this regard, taper limits the useful length of the preform, which means that less optical fiber can be obtained from one preform. Additionally, the properties of an optical fiber may not be constant along the length of the fiber due to the occurrence of taper. Constant optical properties of a fiber are important to a fiber producer, however, because the fiber producer will often have to provide a product certificate with certain guarantees. Typically, the product certificate indicates that each individual part of the optical fiber complies with the specification that is issued, particularly if the optical properties are verified (e.g., by the user).
U.S. Pat. No. 4,741,747, which is hereby incorporated by reference, relates to a method for manufacturing optical fibers in which the reduction of so-called end taper is intended to take place by moving the plasma nonlinearly as a function of time in the region of the reversal point and/or by varying the intensity of the plasma along the length of the glass tube.
U.S. Pat. No. 4,857,091, which is hereby incorporated by reference, relates to a method for manufacturing optical fibers. This patent indicates a number of parameters that influence the axial position of the local deposition zone in relation to the plasma generator, including the periodic variation of the microwave power, periodic variation of the pressure in the substrate tube, and periodic variation of the velocity of the resonator being reciprocated over the tube.
European Patent No. 0,038,982 (and its counterpart U.S. Pat. No. 4,331,462), each of which is hereby incorporated by reference, relate to a method for manufacturing optical fibers, in which the plasma generator is moved along the length of the substrate tube. The plasma generator produces a hot zone, such that the hot zone can be regarded as a so-called “tandem hot zone” that includes at least two zones (e.g., zone I and zone II).
European Patent No. 0,333,580 (and its counterpart U.S. Pat. No. 4,944,244), each of which is hereby incorporated by reference, relate to a method for manufacturing preforms for optical fibers in which a variable power microwave generator is used, but in which no use is made of a non-isothermal plasma being moved back and forth between two points of reversal along the length of the substrate tube. U.S. Pat. No. 4,944,244, discloses a method for manufacturing optical preforms in which the power of the energy source is continuously controlled during the deposition process based upon a signal, which is a function inter alia of deposition of vitreous layers on the interior surface of the substrate tube.
British Patent No. 2,118,165, which is hereby incorporated by reference, relates to a method for manufacturing a preform for an optical fiber in which the velocity of a heat source being moved axially along the length of the substrate tube is in accordance with a specific mathematical equation. The velocity is a function of the position of the heat source along the substrate tube, such that the total deposition thickness of the glass layers is substantially constant along the length of the tube.
U.S. Pat. No. 5,188,648, which is hereby incorporated by reference, relates to a method for manufacturing an optical preform in which the movement of the plasma is interrupted each time the plasma reaches the point of reversal near the gas inlet point of the substrate tube, while the glass deposition continues. The interruption of the plasma movement lasts at least 0.1 second. This patent particularly relates to a reduction of the geometric taper of the preform core.
U.S. Pat. No. 7,068,899 (and its counterpart International Application No. 2004/101458), each of which is hereby incorporated by reference, relate to a deposition method in which the plasma is moved along the length of a hollow substrate tube and is changed in a first end region adjacent to a point of reversal, both as a function of time in the deposition process and as a function of the position in the first end region. The end point of the first end region coincides with the point of reversal, and the starting point is located further away from the point of reversal than the deceleration point. The first end region has a length that is designed to reduce taper in the preform.
Notwithstanding the foregoing, methods for manufacturing preforms designed to improve geometric taper have typically led to increases in optic taper, and vice versa.